P.sup.+ -Poly -Si has been proposed and recommended as the gate material for surface-channel pMOSFETs in deep submicron CMOS (complementary metal-oxide semiconductor) structures [1]. This is because surface-channel pMOSFETs with P.sup.+ -poly-Si gates can improve short-channel and sub-threshold I-V characteristics, and better controllability of the threshold voltage compared with buried-channel pMOSFETs with n.sup.+ -poly-Si gates [2-4].
However, the fast diffusion of boron in poly-Si and within the gate oxide results in the easy penetration of boron through the gate to the underlying silicon substrate [5,6]. The presence of F due to BF.sub.2 implantation enhances the diffusion of B [7,8]. Boron penetration results in instabilities in the device performance, such as positive shifts in the threshold voltage, increases in sub-threshold swing, increases in electron trapping, decreases in low-field hole mobility, and degradation of drive current due to poly-Si depletion in pMOSFETs.
Many methods have been proposed to suppress boron penetration [9-19]. Following the diffusion path of boron diffusion, the first way is to retard the boron diffusion inside the poly-Si. To this end, stacked or modified structures of the poly-Si gate have been proposed [9-10]. The native oxide between the stacked layers acts as the trapping center of F, consequently retarding the enhancement of B diffusion. The stacked structure also exhibits discontinuous poly-Si grain boundaries which disrupt the diffusion path of boron. Nitridation of the stacked poly-Si interface[11], or co-implantation with N, or P is also effective to retard the boron diffusion [12-13]. Utilizing a top oxide on poly-Si to getter the F has also been reported to retard the boron diffusion [14].
The second way is to establish a diffusion barrier at the interface of the poly-Si and oxide. NH.sub.3 nitridation on the gate oxide is feasible [15]. However, the H atom residues during the NH.sub.3 nitridation reduces the resistance to hot carrier stressing. Usually, this method needs an additional reoxidation process to reduce the H atom. Hence, the NH.sub.3 nitridation process is too complex to implement in a real production line and a nitridation process on the oxide surface without H incorporation is worthy of development.
The third way to establish a diffusion barrier at the interface of the poly-Si and oxide is to build a diffusion barrier in the oxide or between the oxide and silicon substrate interface. This can be achieved by nitriding the gate oxide using N O, N.sub.2 O oxidation/annealing[16-19]. In this invention, a novel and simple process to built the diffusion barrier between the poly-Si and oxide was developed by using inductively-coupled nitrogen plasma (ICNP).